1. Field of the Invention
This invention relates in general to telecommunications, and more particularly to a method and apparatus for providing a self-sustaining precision voltage and current feedback biasing loop.
2. Description of Related Art
Global communications continues to demonstrate rapid growth rates. As more people become accustomed to the convenience of electronic mail, web-based facsimile transmission, electronic commerce, telecommuting and high-speed Internet access, the demand on the telecommunications industry to provide adequate bandwidth to provide this type of service also increases. The growth in the number of people using electronic communications will only increase as the price of Internet access and Internet access devices such as personal digital assistants (PDAs), computers, etc.
Today, copper telephone lines service almost all voice traffic and most of the Internet traffic. However, as content rich applications continue to grow, both public and private copper access networks are being challenged. The local portion of the enterprise becomes a major challenge for access providers. To take advantage of the increasingly popular innovations in telecommunications technology, additional telephone lines are being installed in private residences and businesses.
Although analog modems have managed to stretch their potential speed to 56 kilobits per second (kbps), small-office/home-office (SOHO) customers need far greater Internet bandwidth to accommodate multimedia applications ranging form three-dimensional web sites to video conferencing. Analog modems cannot deliver the necessary bandwidth and, therefore, have reached the end of their usefulness.
In response to these developments, communications companies are responding with a variety of digital access solutions, all variants of Digital Subscriber Line (DSL) technology. These DSL technologies differ dramatically in their abilities to address major SOHO applications and the requirements of telephone companies.
DSL technologies are transport mechanisms for delivering high-bandwidth digital data services via twisted-pair copper wires. These copper wires provide the cabling between the telephone company""s central offices and subscribers. DSL technology is a copper loop transmission technology that solves the bottleneck problem often associated with the last mile between Network Service Providers and the users of those network services. DSL technology achieves broadband speeds over ordinary phone wire. While DSL technology offers dramatic speed improvements (up to 7+ Mbps) compared to other network access methods, the real strength of DSL-based services lies in the opportunities driven by multimedia applications required by today""s network users, performance and reliability and economics.
Without such transport mechanisms, subscribers would have to rely on T1 (1.5 Mbps) or E1 (2.0 Mbps) service, which requires the phone company to install expensive new cabling to every location that wants high-speed digital service. The installation costs make T1/E1 service expensive.
The original DSL service was ISDN DSL (ISDL), which was defined in the late 1980s. ISDL provides 160 kbps rates over a single twisted-pair at ranges up to 18,000 feet from the telephone company""s central office. While this service has been deployed to may homes and small businesses all over the world, the demands of multimedia applications are already challenging IDSL""s bandwidth.
Asymmetric Digital Service Line (ADSL) is currently being embraced by residential web surfers for its ability to quickly download music and video files. ADSL refers to modem technology that transforms twisted copper pair (ordinary phone lines) into a pipeline for ultra fast Internet access. As the name suggests, ADSL is not asynchronous transmission, but rather asymmetric digital transmission, i.e., ADSL transmits more than 6 Mbps (optionally up to 8 Mbps) to a subscriber, and as much as 640 kbps (optionally up to 1 Mbps) in the other direction.
ADSL has the ability to increase normal phone line capacity by 99% via a digital coding technique. This extra capacity means that one could simultaneously assess the World Wide Web and use the telephone or send a fax. A user of this technology could have uninterrupted Internet access that is always on-line. This technology also has the potential to be a cost-effective solution for residential customers, telecommuters and small business.
Still, there is a need for symmetric high-speed connection. For example, small businesses have become increasingly dependent on sophisticated voice and data products and services for competing against larger corporations. Until now, the cost of providing small businesses with professional telephony and data services was prohibitive. However, integrated access and virtual public branch exchanges (PBXs) are providing small businesses with voice mail, high-speed Internet access, multiple business lines and sufficient capabilities for telecommuters.
As mentioned above, symmetric services were traditionally delivered by T1 and E1 lines. Within the DSL family, HDSL has long been used to provision T1 lines because its long reach requires regeneration-signal boosting-only every 12,000 feet, compared with every 4,000 feet for other T1 provisioning techniques. In fact, HDSL""s ability to simplify and cheapen T1 deployment has made HDSL by far the most established of the DSL technology family.
As an inexpensive and flexible replacement for leased T1 lines, the HDSL2 standards are eagerly awaited by the DSL industry. HDSL2 replaces the aging HDSL standard that required two copper pairs. HDSL2 uses only one copper pair and is potentially rate adjustable. HDSL2 , which is being developed within the framework of the American National Standards Institute (ANSI, New York), promises to make HDSL more compelling in two ways. While HDSL was a proprietary technique-modems at the central office (CO) and the customer premises had to come from the same vendor-HDSL2 will be an interoperable standard in which modems can be mixed. Perhaps the biggest selling point of HDSL 2, however, is that it can use one pair of copper wires instead of HDSL""s two. Network service providers thus have a choice. HDSL and one-pair HDSL2 have about the same reach, while two-pair HDSL2 adds as much as another 4,000 feet of reach, depending on the gauge of copper and other conditions. Hoping to propel the new DSL technology into the business arena, eight chip makers and OEMs have formed a consortium for the HDSL2 standard.
An HDSL2 transceiver includes a framer, a data pump and an analog interface for coupling to the twisted-pair line. In the transmit function, the framer accepts a digital signal and outputs to the data pump a serial digital signal that includes the data payload plus an HDSL2 overhead. In the receive function, the framer receives HDSL frames from the data pump.
The data pump includes a transceiver and an analog front end that receives the HDSL frames serially from the framer. The transceiver converts the HDSL frames into a transmit signal by first converting the HDSL frames into symbols. Typically, a modulator, such as a trellis code modulator (TCM) encodes the symbols into a pulse amplitude modulation (PAM) signal. The signal is further processed to condition and filter the PAM signal. The analog front end provides pulse shaping to analog signals. This process is reversed in the receive channel with echo cancellation provided to cancel most of the echoed transmit signal.
As mentioned, the analog front end includes a transmit and a receive channel. In the transmit channel, the analog front end receives a pulse width modulated digital data stream from the transceiver. The parallel digital data is converted to a serial analog signal via a parallel-to-serial/digital-to-analog D/A converter. A switched-capacitor circuit filter shapes the analog signal to meet specific spectral templates. The receive channel consists of an automatic gain control (AGC) stage and an analog-to-digital (A/D) converter. The AGC stage sets the amplitude to the optimum level to prevent saturation of the A/D converter.
Implementation of high precision and low noise A/D convertors and D/A convertors requires the associated voltage and current references be very accurate and low noise in nature. Accuracy of voltage reference is required to accurately transmit the desired power to the line, independent of process, voltage and temperature conditions. Since the voltage reference is used by both the A-to-D and D-to-A convertors, the reference is also required to be very low noise in nature otherwise it degrades the Signal-to-Noise ratio of the signal processing paths. Based on these requirements, the voltage reference is often implemented in the form of a bandgap reference.
The current reference is also desired to be very accurate over process, voltage and temperature conditions. Wide current tolerances, e.g., xc2x125%, will require all the operational amplifiers to meet performance specifications over the worst case current tolerance, which would lead to more power consumption and overdesign. Excessive noise on the current reference can also show up in the output spectrum of received and transmit signals. Hence the current reference is also desired to be very accurate, e.g., xc2x15%, with very low noise on it. The current reference is implemented using the available accurate and low noise bandgap voltage and applying it to an external low tolerance resistor. Based on accurate voltage and resistor, the derived current is accurate. This current is filtered and then mirrored for use in all the other blocks. This reference current generator is referred to as Master Bias Current Generator (MBCG).
In such a scheme as described above, the bandgap reference voltage generator also requires a reference bias current for its own operation. This reference can be locally generated or can come from the master bias current generator (MCBG) on the chip. The local current will have wide tolerance over process, temperature and voltage, e.g., xc2x1,35-50%, and will not be low noise. To provide a low noise local current requires excessive filtering. Thus, the bandgap circuit must work properly under large variations of current, leading to more typical power consumption which is undesirable. On the other hand, if the current from master bias current generator is used, then the bandgap circuit will get a very accurate and low noise current. This current is low in noise for two reasons: it uses bandgap reference for a reference voltage, which itself is required to be low noise, and secondly, substantial filtering is provided in current mirrors since the A-to-D and D-to-A require low noise currents. Hence it is desirable to use the current bias for the bandgap circuit also.
This presents a problem because a current from MCBC is used to bias the bandgap, and the voltage from bandgap is used to bias and generate current in the MCBG. This creates a positive feedback loop which is feeding on to itself. This loop can have both start-up and sustained operation problems in form of oscillations.
It can be seen then that there is a need for initiating such a loop at the time of power-up.
It can also be seen then that there is a need for a method and apparatus for providing a self-sustaining precision voltage and current feedback biasing loop.
It can also be seen then that there is a need for a feedback biasing loop having loop dynamics that are chosen such that the gain of the positive feedback loop is less than one so that the loop will not oscillate under normal operation after power-up.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for providing a self-sustaining precise voltage and current feedback biasing loop.
The present invention solves the above-described problems by providing a circuit for initially biasing the bandgap and master bias current generator at startup.
A method in accordance with the principles of the present invention includes determining whether a generated voltage satisfies a threshold condition, establishing a first reference voltage for generating a bias current when the generated voltage does not satisfy the threshold condition and establishing a second reference voltage for generating the bias current when the generated voltage satisfies the threshold condition, wherein the bias current is used to create the generated voltage.
Other embodiments of a method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the determining whether a generated voltage satisfies a threshold condition further comprises comparing the generated voltage to a predetermined comparison voltage.
Another aspect of the present invention is that the establishing the first reference voltage includes generating a control voltage when the generated voltage is less than the predetermined comparison voltage and using the control voltage to turn off a first device for establishing a second reference voltage and to turn on a second device, the turning on of the second device creating the first reference voltage.
Another aspect of the present invention is that the establishing the second reference voltage includes driving a first device with the generated voltage to turn on the first device and creating the second reference voltage in response to turning on the first device.
Another aspect of the present invention is that the generating the bias current further includes processing the first or second reference voltage to produce a current control voltage, driving a third device with the current control voltage to create a first current, and mirroring the first current to produce the bias current.
The present invention may be embodied in a feedback biasing loop for providing highly accurate and low noise voltage and current signals to components, such as analog-to-digital and digital-to-analog converters. Such a feedback biasing loop includes a bandgap reference voltage generator for generating a bandgap voltage output, a master bias current generator, coupled to the bandgap reference voltage generator, for generating a bias current in response to the bandgap voltage output, the bias current being provided to the bandgap reference voltage generator by a current feedback loop for controlling the generation of the bandgap voltage output and an initialization device, coupled to the bandgap reference voltage generator, for ensuring proper start-up of the current feedback loop. The feedback biasing loop may also be implemented in an analog front end of an HDSL2 system.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.